The operating temperature within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature capabilities have been achieved through the development of iron, nickel and cobalt-based superalloys and the use of environmental coatings capable of protecting superalloys from oxidation, hot corrosion, etc., but coating systems continue to be developed to improve the performance of the materials.
In the compressor portion of an aircraft gas turbine engine, atmospheric air is compressed to 10–25 times atmospheric pressure, and adiabatically heated to 800°–1250° F. in the process. This heated and compressed air is directed into a combustor, where it is mixed with fuel. The fuel is ignited, and the combustion process heats the gases to very high temperatures, in excess of 3000° F. (1650° C.). These hot gases pass through the turbine, where airfoils fixed to rotating turbine disks extract energy to drive the fan and compressor of the engine, and the exhaust system, where the gases supply thrust to propel the aircraft. To improve the efficiency of operation of the aircraft engine, combustion temperatures have been raised. Of course, as the combustion temperature is raised, steps must be taken to prevent thermal degradation of the materials forming the flow path for these hot gases of combustion.
Aircraft gas turbine engines have a so-called High Pressure Turbine (HPT) to drive the compressor. The HPT is located immediately aft of the combustor in the engine layout and experiences the highest temperature and pressure levels (nominally −3000° F. (1850° C.) and 300 psia, respectively) developed in the engine. The HPT also operates at very high rotational speeds (10,000 RPM for large high-bypass turbofans, 50,000 for small helicopter engines). There may be one or two stages of airfoils in the HPT. In order to meet life requirements at these levels of temperature and pressure, HPT components are always air-cooled and are constructed from advanced alloys.
While a straight turbojet engine and a low-bypass turbofan engine will usually have only one turbine (an HPT), most engines today are of the high-bypass turbofan or turboprop type and require one (and sometimes two) additional turbine stage(s) to drive a fan or a gearbox. This stage is called the Low Pressure Turbine (LPT) and immediately follows the HPT in the engine layout. Since substantial pressure drop occurs across the HPT, the LPT operates with a much less energetic fluid and will usually require several stages (usually up to six) to extract as much power as possible.
Components formed from iron, nickel and cobalt-based superalloys cannot withstand long service exposures if located in certain sections of a gas turbine engine, such as the LPT and HPT sections. A common solution is to provide such components with an environmental coating that inhibits oxidation and hot corrosion. Coating materials that have found wide use for this superalloy generally include diffusion aluminide coatings. These coatings are generally formed by such methods as diffusing aluminum deposited by chemical vapor deposition (CVD), slurry coating, pack cementation, above-pack, or vapor (gas) phase aluminide (VPA) deposition into the superalloy.
A diffusion aluminide coating generally has two distinct zones, the outermost of which is an additive layer containing an environmentally resistant intermetallic generally represented by MAl, where M is iron, nickel, cobalt, a noble metal or combinations thereof depending on the substrate material. Beneath the additive layer is a diffusion zone comprising various intermetallic and topologically close packed (TCP) phases that form during the coating reaction as a result of compositional gradient(s) and changes in elemental solubility in the local regions of the substrate. During high temperature exposure in air, a thin protective aluminum oxide (alumina) scale or layer that inhibits oxidation of the diffusion coating and the underlying substrate forms over the additive layer.
Currently, the majority of HPT blades are protected from the environment with PtAl environmental coatings or thermal barrier systems (a bond coat with ceramic top coat). Greater performance is being sought for these systems to increase turbine temperature capability and improve repairability of components. Different coating compositions and processes are required to achieve this capability. However, the new processes being used are limited to line-of-sight regions of the blade. This limitation highlights the need for cost-effective processes to apply coatings in the non line-of-sight areas of the components getting these new overlay coatings.
Recently, high Al, primarily beta phase nickel aluminum (β-NiAl) based coatings, which contain additions of chromium (Cr) and reactive elements such as zirconium (Zr), hafnium (Hf), and yttrium (Y), have been developed to serve as bond coatings for thermal barrier coatings for gas turbine engine components. Such high Al, primarily beta phase nickel aluminum (β-NiAl) based coatings are disclosed in U.S. Pat. No. 6,153,313, issued Nov. 28, 2000; U.S. Pat. No. 6,255,001 B1, issued Jul. 3, 2001; U.S. Pat. No. 6,291,084 B1, issued Sep. 18, 2001, which are assigned to General Electric Company, the assignee of the present invention, and which are hereby incorporated by reference. Unfortunately, due to the nature of the processes used to apply these β-NiAl-base coatings, the bond coatings cannot be applied to the interior cooling passageways of components that use film cooling, nor can the processes be used to coat exterior portions of the components that are shadowed due the complex geometry of such components. If standard chemical vapor deposition or vapor phase aluminization processes are used to coat the interior cooling passageways and/or the exterior shadowed surfaces of the components, then excess aluminum is deposited into the β-NiAl coatings. The rate at which a diffusion aluminide coating develops on a substrate is dependent in part on the substrate material, the amount and type of donor material, the amount and type of activator used and the temperature of the aluminization process.
Without the deposition of a diffusion aluminide coating into the sections of the component that cannot be coated with the β-NiAl-base coatings, the operable life of the component will be severely limited. However, excess aluminization adversely affects the performance of the β-NiAl-base layers.
What is needed is a modified chemical vapor deposition (CVD), vapor phase aluminiding (VPA), or above the pack aluminization process that will aluminide the desired bare regions of the airfoil (internal and external), but that will not substantially affect the surface chemistry of the pre-deposited coatings. The present invention provides an aluminization process with novel process parameters for aluminizing non-line-of-sight regions of components that have primarily β-NiAl coatings, or other aluminide coatings, already applied.